In-Situ Microstructure Tuning: Unlocking Next-Generation LPBF Efficiency

Removing Heat Treatments in Metal Additive Manufacturing

Controlling precipitation strengthening in stainless steel during LPBF

Metal additive manufacturing (AM), particularly Laser Powder Bed Fusion (LPBF), allows for the production of highly complex geometries with fine precision. However, scaling metal AM economically remains a challenge. Conventional post-processing and thermal treatments can account for up to 27% of total manufacturing costs and as much as 50% of the associated carbon footprint due to high energy consumption and long cycle times. To overcome these economic and environmental bottlenecks, researchers are leveraging Intrinsic Heat Treatments (IHT), the repeated thermal cycles experienced as consecutive laser passes heat adjacent material during printing. By optimising laser processing parameters, IHT can tailor local microstructures directly during the build, eliminating the need for energy-intensive post-build heat treatments. 

To systematically analyse the influence of laser parameters on microstructure evolution, experimental trials were conducted using the AconityMINI at the Royce Discovery Centre. 

An array of 13 cuboid-shaped pillars were fabricated from 17-4 PH stainless steel powder. Each individual pillar was printed using a unique combination of laser power and scanning speed, directly altering the volumetric energy density applied during consolidation. By systematically varying these input parameters, the team generated distinct thermal histories and overlapping heat-affected zones within each specimen. 

Advanced characterisation techniques were subsequently employed to evaluate the resulting microstructural changes, focusing on the in-situ precipitation kinetics of copper-rich phases and phase transformations across the pillar set.

This in-situ microstructure engineering approach offers transformative potential across high-performance sectors such as aerospace, automotive, energy, and biomedical manufacturing. By eliminating or drastically reducing post-processing heat treatment cycles, manufacturers can reduce part lead times, lower energy consumption, and significantly decrease embodied carbon emissions per component. 

Furthermore, controlling microstructural features at the local level enables the fabrication of functionally graded materials, where mechanical properties like hardness or ductility are tailored point-by-point within a single monolithic part. On an industrial scale, this strategy enhances the cost-competitiveness and sustainability of metal additive manufacturing, accelerating its adoption for critical structural components.

Associated paper: Effect of energy density on microstructure control in precipitation hardening steel during powder bed fusion